Copper spheres and to a lesser degree spheres of other metals are an increasingly important intermediary product in the electronic circuit board industry. In particular, they are used in the emerging technology of ball grid arrays where devices are attached to circuit boards by the use of solder balls. For instance, in order to improve thermal and electrical conductivity, new developments make use of spheres comprising pure copper spheres coated with solder, typically a 63% tin 37% lead solder composition. Also, pluggable ball grid arrays are known in which devices having metal balls attached thereto can be plugged into sockets by application of pressure. Copper spheres with hard surfaces are well suited for this application since the hard surfaces are resistant to the rubbing contact of socket springs. Demonstrated techniques for making copper spheres include gas atomization of a stream of molten copper, vibration of molten copper through a multiple orifice die, and grinding and polishing of chopped wire.
Two key requirements for spheres used in applications such as ball grid arrays are that they be uniformly sized and almost perfectly spherical to ensure consistency of contact between the various connecting points on an electronic device and the circuit board to which it is attached. A typical sphere diameter requirement such as 712.+-.25 micrometers is difficult if not impossible to achieve directly by any of the aforementioned production processes. Gas atomization produces a wide sphere size distribution yielding a low recovery. The multiple orifice die vibration technique also produces wider than required sphere size distribution which is very difficult to centre on the narrow size interval required. This leads to low sphere recovery in the required interval of particle sizes.
Product sphericity, which is the average ratio of sphere maximum diameter (Dmax) relative to sphere minimum diameter (Dmin) must be less than 1.05, i.e. the difference between Dmax and Dmin must be less than 5% for electronic applications.
Grinding and polishing of chopped wire to produce spheres of desired sphericity not only is substantially more expensive than the above-mentioned two techniques but also it is very difficult to accomplish because of the softness of metal particles such as copper particles. Also, the particle softness leads to heavy surface contamination from the grinding and polishing media, rendering the product unusable for electronic applications.
It should be understood that the sphericity of the final product can be achieved only if the spheres before electroplating are very close to the sphericity of the required final product. That is to say, electroplating cannot substantially change the particle sphericity. If nonspherical starting material is used, it has to be made spherical either before or after electroplating by such techniques as grinding or polishing.
It is a principal object of the present invention to provide a method for producing uniformly sized metal spheres which when coated with solder or used without a solder coating will meet the requirements of electronic connective applications such as use in ball grid arrays. It will be understood that although the description of the invention will proceed with reference to the coating of core spheres or like particles by copper electroplating, the technology disclosed in this application could be based on various other techniques for coating the core spheres in addition to electroplating such as electroless plating, mechanical plating, physical vapor deposition and chemical vapor deposition with metals other than copper. It will also be understood that the term "metal" used herein will include both metals and metal alloys.
It is a further object of the invention to produce radially differentiated, composite metal spheres of a uniform size that have a hardened surface to improve their utility, such as for use in ball grid arrays, and have an acceptable high level of sphericity. The invention is not limited to the production of spheres with a hardened surface, and may include spheres with a unique combination of bulk and surface properties achieved by the processes described herein.